Chemical composition of virus

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Most DNA viruses Fig. The papovaviruses, comprising the polyoma- and papillomaviruses, however, have circular DNA genomes, about 5. Three or 2 structural proteins make up the papovavirus capsid: in addition, nonstructural proteins are encoded that are functional in virus transcription, DNA replication and cell transformation.

Single-stranded linear DNA, 4—6 kb in size, is found with the members of the Parvovirus family that comprises the parvo-, the erythro- and the dependoviruses. The virion contains 2—4 structural protein species which are differently derived from the same gene product see Ch. The adeno-associated virus AAV, a dependovirus is incapable of producing progeny virions except in the presence of helper viruses adenovirus or herpesvirus.

It is therefore said to be replication defective. Circular single-stranded DNA of only 1. The isometric capsid measures 17 nm and is composed of 2 protein species only.

On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species. More than 30, different virus isolates are known today and grouped in more than 3, species, in genera and 71 families. Viral morphology provides the basis for grouping viruses into families. A virus family may consist of members that replicate only in vertebrates, only in invertebrates, only in plants, or only in bacteria.

Certain families contain viruses that replicate in more than one of these hosts. This section concerns only the 21 families and genera of medical importance. Besides physical properties, several factors pertaining to the mode of replication play a role in classification: the configuration of the nucleic acid ss or ds, linear or circular , whether the genome consists of one molecule of nucleic acid or is segmented, and whether the strand of ss RNA is sense or antisense.

Also considered in classification is the site of viral capsid assembly and, in enveloped viruses, the site of nucleocapsid envelopment. Table lists the major chemical and morphologic properties of the families of viruses that cause disease in humans. The use of Latinized names ending in -viridae for virus families and ending in -virus for viral genera has gained wide acceptance. The names of subfamilies end in -virinae. Vernacular names continue to be used to describe the viruses within a genus.

In this text, Latinized endings for families and subfamilies usually are not used. Table shows the current classification of medically significant viruses. In the early days of virology, viruses were named according to common pathogenic properties, e. From the early s until the mids, when many new viruses were being discovered, it was popular to compose virus names by using sigla abbreviations derived from a few or initial letters.

Thus the name Picornaviridae is derived from pico small and RNA; the name Reoviridae is derived from respiratory, enteric, and orphan viruses because the agents were found in both respiratory and enteric specimens and were not related to other classified viruses; Papovaviridae is from papilloma, polyoma, and vacuolating agent simian virus 40 [SV40] ; retrovirus is from reverse transcriptase; Hepadnaviridae is from the replication of the virus in hepatocytes and their DNA genomes, as seen in hepatitis B virus.

Hepatitis A virus is classified now in the family Picornaviridae, genus Hepatovirus. Although the current rules for nomenclature do not prohibit the introduction of new sigla, they require that the siglum be meaningful to workers in the field and be recognized by international study groups. Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization.

Hepatitis E virus, the Norwalk virus and similar agents see Ch. The fatal transmissible dementias in humans and other animals scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans see Ch.

The agents causing transmissible subacute spongiform encephalopathies have been linked to viroids or virinos i. Some of the transmissible amyloidoses show a familial pattern and can be explained by defined mutations which render a primary soluble glycoprotein insoluble, which in turn leads to the pathognomonic accumulation of amyloid fibers and plaques.

The pathogenesis of the sporadic amyloidoses, however, is still a matter of highly ambitious research. Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. General Concepts Structure and Function Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Classification of Viruses Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication.

Nomenclature Aside from physical data, genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses, including the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite.

Structure and Function Viruses are inert outside the host cell. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication. Morphology Helical Symmetry In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.

It has axes of two-, three-, and fivefold rotational symmetry, passing through its edges, faces, and vertices, respectively see Fig. The icosahedron is the optimum solution to the problem of constructing, from repeating subunits, a strong structure to enclose a maximum volume.

Features of icosahedral structure. A regular icosahedron viewed along twofold A , threefold B , and fivefold C axes of symmetry. In negatively stained electron micrographs, virions may appear hexagonal in outline upper row or apparently spherical middle row. Various clusterings of capsid polypeptides give characteristic appearances of the capsomers in electron micrographs lower row. For example, they may be arranged as 60 trimers D , capsomers being then difficult to define, as in poliovirus; or they may be grouped as 12 pentamers and 20 hexamers E , which form bulky capsomers as in parvoviruses; or as dimers on the faces and edges of the triangular facets F , producing an appearance of a bulky capsomer on each face, as in caliciviruses.

Only certain arrangements of the repeating morphological units, the capsomers, can fit into the faces, edges, and vertices. In adenovirus particles, for example, capsomers on the faces and edges bond to six neighboring capsomers and are called hexamers; those at the vertices bond to five neighbors and are called pentamers Fig. In some viruses both hexamers and pentamers consist of the same polypeptide; in others they are different.

Some possible arrangements of capsomers are shown in Fig. A Icosahedral structure of adenovirus capsid. At each of the 12 vertices there is a penton base capsomer from which projects a fiber with a small terminal knob; each of the 20 triangular facets contains 18 identical hexon capsomers, of which 6 are unshared and 12 shared with adjacent facets.

The capsid encloses a protein core with which the DNA is associated. B Envelope of influenza virus family: Orthomyxoviridae. The peplomers are of two morphological types: the hemagglutinin is a rod-shaped trimer and the neuraminidase is a stud-shaped tetramer.

Both are embedded in lipid, beneath which there is a matrix protein; this lipoprotein envelope encloses a helical nucleocapsid. C Nucleocapsid of parainfluenza virus family: Paramyxoviridae. The RNA is wound within and protected by a helix composed of identical capsomers. A and B, courtesy Dr. Wrigley; C, courtesy Dr. In a practical sense, the examination of negatively stained icosahedral virions in the electron microscope, and analysis of their capsomer arrangement, can often provide immediate and unambiguous information for the identification of a virus as a member of a known family—or, in very rare instances, as a candidate prototype for a new family.

For example, the visualization of a nonenveloped virion with a row of four hexamers in line between vertex pentamers would identify a virus as an adenovirus Plate A. The recent demonstration by X-ray crystallography of the atomic resolution structure of two picornaviruses poliovirus and rhinovirus has provided a remarkable insight into the organization and assembly of their virions, the location of the antigenic sites involved in neutralization, and aspects of their penetration into cells.

Similar detail can be expected as these new technical capabilities are applied to other viruses and to problems of replication, assembly, and pathogenesis. The nucleocapsids of several RNA viruses have a different type of symmetry: the capsomers and nucleic acid molecule s self-assemble as a helix Fig.

In all such viruses each capsomer consists of a single polypeptide molecule. The plant viruses with helical nucleocapsids are rod shaped and naked nonenveloped. However, in all animal viruses helical nucleocapsids are wound into a coil and enclosed within a lipoprotein envelope see Plate , possibly to give the very long nucleocapsids stability. These peplomers can often be seen clearly in electron micrographs as projections from the outer surface of the envelope Plate B.

The other kind of envelope protein, matrix protein, is nonglycosylated and is found on the inside of the envelope of virions of several families; it provides added rigidity. The envelope of rhabdoviruses is closely applied to a bullet-shaped matrix protein that encloses a helical nucleocapsid.

Arenaviruses, bunyaviruses, and coronaviruses have no matrix protein and consequently are rather more pleomorphic than other enveloped viruses. Envelopes are not restricted to viruses of helical symmetry; some icosahedral viruses ranaviruses, African swine fever virus, herpesviruses, togaviruses, flaviviruses, and retroviruses have envelopes. The infectivity of most enveloped viruses depends on the integrity of the envelope, but some poxviruses have an envelope which is not necessary for infectivity.

The essential components of infectious viral particles are nucleic acid the genome and protein. In addition, all enveloped viruses contain lipid in the envelope and carbohydrate in their glycoprotein peplomers as well as that in the nucleic acid.

The largest and most complex viruses poxviruses, ranaviruses, and African swine fever virus also have lipids associated with other parts of the virion. Any particular virus contains only a single kind of nucleic acid. All viral genomes are haploid, i. Since the genomes of many of the smaller animal viruses have been sequenced, and there are now no insuperable technical impediments to the sequencing of any viral genome.

By , the largest genome to be completely sequenced was that of a herpesvirus EB virus , which consist of , base pairs kilobase pairs, kbp. When carefully extracted from the virion, the nucleic acid of viruses of certain families of both DNA and RNA viruses is infectious; i.

In other cases, the isolated nucleic acid is not infectious even though it contains all the necessary genetic information. The genome of all DNA viruses consists of a single molecule, which is double-stranded except in the case of the parvoviruses, and may be linear or circular. The DNA of papovaviruses and hepadnaviruses is circular.

Within the virion, the circular DNA of the papovaviruses, like that of mitochondria and bacterial plasmids, is a supercoiled circle, known as a superhelix Plate A. When an enzyme relieves the tension by introducing a nick into one strand, the molecule becomes a relaxed circle Plate B. One strand of the circular DNA of hepadnaviruses is shorter than the other; the genome is thus only partially double-stranded. A When it is isolated from the virions, most of the DNA occurs as a double-stranded, supercoiled, circular molecule superhelix.

B If one of the DNA strands is nicked, the superhelix becomes a circle. Most of the linear DNAs from viruses of other families have characteristics which enable them to adopt a circular configuration temporarily, presumably during replication. The two strands of poxvirus DNA are covalently cross-linked at each end, so that on denaturation, the molecule becomes a large single-stranded circle Fig. The linear dsDNA of some herpesviruses and the linear ssRNA of retroviruses contains repeat sequences at the ends of the molecule.

In the case of the adenoviruses, these terminal repeats are inverted; hence, even without enzymatic digestion, denatured molecules self-anneal to form single-stranded circles Fig. Specialized arrangements at the termini of linear DNA viral genomes. A Adenovirus DNA has inverted terminal repeats, with a covalently linked protein located at each end of the molecule. B Herpes simplex virus DNA consists of two covalently linked components, long L and short S , each of which consists of a large unique sequence U L and U S , respectively flanked by inverted repeats.

Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid. The envelope is composed of two lipid layers interspersed with protein molecules lipoprotein bilayer and may contain material from the membrane of a host cell as well as that of viral origin.

The virus obtains the lipid molecules from the cell membrane during the viral budding process. However, the virus replaces the proteins in the cell membrane with its own proteins, creating a hybrid structure of cell-derived lipids and virus-derived proteins. Many viruses also develop spikes made of glycoprotein on their envelopes that help them to attach to specific cell surfaces. Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins.

While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA. Most viruses maintain all their genetic information with the single-stranded RNA. There are two types of RNA-based viruses.

In most, the genomic RNA is termed a plus strand because it acts as messenger RNA for direct synthesis translation of viral protein. A few, however, have negative strands of RNA. In these cases, the virion has an enzyme, called RNA-dependent RNA polymerase transcriptase , which must first catalyze the production of complementary messenger RNA from the virion genomic RNA before viral protein synthesis can occur.

The Influenza Flu Virus - Next to the common cold, influenza or "the flu" is perhaps the most familiar respiratory infection in the world. In the United States alone, approximately 25 to 50 million people contract influenza each year.

The symptoms of the flu are similar to those of the common cold, but tend to be more severe. Fever, headache, fatigue, muscle weakness and pain, sore throat, dry cough, and a runny or stuffy nose are common and may develop rapidly. Gastrointestinal symptoms associated with influenza are sometimes experienced by children, but for most adults, illnesses that manifest in diarrhea, nausea, and vomiting are not caused by the influenza virus though they are often inaccurately referred to as the "stomach flu.

Since that time, a tremendous amount of research focusing upon the causative agent of AIDS has been carried out and much has been learned about the structure of the virus and its typical course of action.